Electromagnetic filter assemblies

ABSTRACT

An electromagnetic filter assembly includes complimentary first and second blocks ( 1 ) and ( 2 ) defining two conjoined sections ( 10 ), ( 11 ) of an electromagnetic waveguide duct. A shim-like electromagnetic filter element ( 3 ) is held between the blocks and contains a series of holes ( 14 ) defining bridges ( 15 ) which traverse the waveguide duct to form poles of an electromagnetic filter. The filter element includes integral tuning projections ( 20 ) which extend into waveguide duct between the bridges. By adjusting the position of the filter element ( 3 ) between the blocks ( 1 ), ( 2 ) it is possible to vary the extent to which the tuning projections ( 20 ) project into the waveguide duct. The arrangement thus facilitates the repeated, continuous adjustment and variation of the electrical characteristics of devices such as filters, diplexers and multiplexers.

TECHNICAL FIELD OF THE INVENTION

This invention relates to electromagnetic filter assemblies such as areused in filters, diplexers and multiplexers. More particularly, theinvention relates to the tuning of such filters.

BACKGROUND

Filters, diplexers, multiplexers and other devices which incorporate anelectromagnetic filter assembly disposed in an electromagnetic waveguidemay be manufactured in the form of two blocks or plates with half of thewaveguide duct machined in each plate and one or more a thin metalsheets or shims with appropriate cut-outs sandwiched between the plates.The cut-outs in the or each shim are produced in such a way that whenthe parts are assembled, the cut-outs form a series of thin metalbridges which traverse the waveguide duct. The axial thickness andspacing of the bridges, and the number of bridges, determine thefrequency characteristics of the filter.

The current known art of tuning waveguides is to use screws whichpenetrate into the waveguide duct, positioned along the centre line ofthe waveguide. This achieves minimum insertion loss and maximum tuningeffect. However, such an arrangement is not practical with metal shimfilters since the tuning screws must be clear of the shim, and placingthe screws clear of the shim increases the insertion loss of the filter.Furthermore, in devices which incorporate two or more filter assembliesuse of tuning screws may require the tuning of each filter cavity andthus has the disadvantage of requiring skilled operators to carry outthe task, which may be time consuming.

Another known way of tuning such devices is to form dents in the wallsof the waveguide duct, but this has the disadvantage of being timeconsuming, difficult to achieve and irreversible.

The present invention seeks to provide a new and inventive form oftuning method for such devices, which does not involve any of the abovemethods and is inexpensive, fast, simple to achieve and manufacture,repeatable, low loss and reversible.

SUMMARY OF THE INVENTION

The present invention proposes an electromagnetic filter assembly whichincludes complimentary first and second blocks defining two conjoinedsections of an electromagnetic waveguide duct, and a shim-likeelectromagnetic filter element which is held between the blocks andcontains a series of holes which define bridges which traverse thewaveguide duct to form poles of an electromagnetic filter,

-   -   characterised in that        the filter element includes integral tuning projections which        extend into waveguide duct between the bridges.

By adjusting the position of the filter element between the blocks it ispossible to vary the extent to which the tuning projections project intothe waveguide duct. The arrangement thus facilitates the repeated,continuous adjustment and variation of the electrical characteristics ofdevices such as filters, diplexers and multiplexers.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred totherein are included by way of non-limiting example in order toillustrate how the invention may be put into practice. In the drawings:

FIG. 1 is a general exploded view of an electromagnetic filter assemblyin accordance with the invention;

FIG. 2 shows the three main components of the assembly, each in planview; and

FIGS. 3 to 5 each show the components partly assembled in threedifferent adjustment conditions.

DETAILED DESCRIPTION OF THE DRAWINGS

The drawings show an electromagnetic waveguide filter assembly of thekind which may be used in various devices such as filters, diplexers andmultiplexers.

FIG. 1 shows the main components of the assembly, namely complimentaryfirst and second blocks 1, 2 and a shim-like electromagnetic filterelement 3. As can also be seen in FIG. 2, the blocks 1 and 2 are formedof metal and contain two complimentary machined channels 10 and 11which, when the blocks are brought together, become conjoined to form anelectromagnetic waveguide duct. In this example the duct is ofsubstantially rectangular cross-section although those skilled in theart will appreciate that the duct can be of any other appropriate shapesuch as round, oval, elliptical etc. The shim 3 is held between theblocks, which may be connected together by any suitable means such asbolts, clamps etc. By way of example, screws 13 may pass through holes 6in the first block 1 to engage in threaded apertures 7 in the secondblock 2. The shim 3 is provided with corresponding slots 8 to receivethe screws 5, extending transverse to the waveguide duct.

The shim 3 is formed from a conductive metal foil and contains a row ofrectangular holes 14 which form the poles of an electromagnetic filterand which define a series of foil bridges 15. When the blocks 1 and 2are brought together as in FIG. 1 the filter element 3 is held betweenthe blocks with the holes 14 disposed in the waveguide duct such thatthe bridges 15 extend transversely across the duct. It will be noted inFIG. 2 that the dimension D of the holes 14, which extends transverse tothe axis A of the waveguide, is greater than the transverse width W ofthe waveguide. One longitudinal edge of each hole 14 is provided with anintegral foil projection 20 (shown as a rectangular tab) which isdisposed mid-way between adjacent bridges 15.

By adjusting the transverse position of the filter element 3 between theblocks 1 and 2 it is possible to vary the extent to which the tuningtabs 20 project into the waveguide duct. Thus, at one end of the tuningrange, shown in FIG. 3, the tabs 20 may be completely withdrawn betweenthe blocks 1 and 2 so that they do not project beyond the opposed edges30 of the waveguide duct. In the mid tuning range the tabs projectpart-way into the duct as shown in FIG. 4, and at the other end of thetuning range the tabs extend fully into the duct as shown in FIG. 5. Thearrangement thus allows continuous adjustment and variation of theelectrical characteristics of the filter.

The movement of the shim may be achieved by one or more posts 40 (FIG.2) which are rotatably mounted in either of the blocks 1, 2 witheccentric fingers 41 located in slots 42 in the shim 3 extendingparallel to the waveguide duct. The tuning may, but not exclusively, beachieved in the following manner. Screws 13 holding the top and bottomplates together are loosened sufficiently to just allow the shim toslide. The tuning post (or posts) are turned to slide the shimtransverse to the duct and tune the filter to the requiredspecification. The screws holding the top and bottom plates together arethen tightened to the required torque. Small adjustments to the tuningcan be made by repeating the above steps. When completed the fixingscrews and shim adjusting post(s) may be fixed in place by a suitablemeans, e.g. glue etc.

It will be appreciated that the features disclosed herein may be presentin any feasible combination. Whilst the above description lays emphasison those areas which, in combination, are believed to be new, protectionis claimed for any inventive combination of the features disclosedherein.

1. An electromagnetic filter assembly which includes complimentary firstand second blocks (1, 2) defining two conjoined sections (10, 11) of anelectromagnetic waveguide duct, and a shim-like electromagnetic filterelement (3) which is held between the blocks and contains a series ofholes (14) which define bridges (15) which traverse the waveguide ductto form poles of an electromagnetic filter, characterised in that thefilter element includes integral tuning projections (20) which extendinto the waveguide duct between the bridges; and in that said duct has alongitudinal axis and said holes have a transverse dimension, transverseto said longitudinal axis, which is greater than the correspondingtransverse width of the waveguide duct.
 2. An electromagnetic filterassembly in accordance with claim 1 wherein said transverse dimension ofsaid holes is such that said tuning projections can be completelywithdrawn from said waveguide duct.
 3. An electromagnetic filterassembly in accordance with claim 1 wherein said tuning projections aredisposed substantially mid-way between adjacent bridges.
 4. Anelectromagnetic filter assembly in accordance with claim 1 wherein saidholes are substantially rectangular.
 5. An electromagnetic filterassembly in accordance with claim 1 wherein said tuning projections aresubstantially rectangular.